1. Field of the Invention
The invention relates to an exhaust gas purifying system for an internal combustion engine.
2. Description of the Related Art
It has widely been known in the art to use nitrogen oxide absorbing materials for controlling NOx emission level. Such a nitrogen oxide absorbing material, installed in an exhaust line, on one hand, absorbs nitrogen oxides (NOx) in an exhaust gas while an air-fuel ratio of the exhaust gas is higher and, on the other hand, releases the nitrogen oxides (NOx) while the concentration of oxygen (O.sub.2) in the exhaust gas is lowered. The nitrogen oxides (NOx) released from the nitrogen oxide absorbing material are then reduced. In the case where such a nitrogen oxide absorbing material is installed as a catalyst in an exhaust system of a vehicle which is used in a specific countries or regions where a fuel and an engine oil ontain sulfur, the nitrogen oxide absorbing material has a property of easily absorbing sulfur oxides (SOx) rather than nitrogen oxides (NOx) in the exhaust gas. The nitrogen oxide absorbing material poisoned as a result of sulfur oxide absorption suffers a significant decline in efficiency of nitrogen oxide absorption.
Japanese Unexamined Patent Publication No. 6-272541 teaches that a nitrogen oxide absorbing material containing barium oxide is changed into barium sulfate (BaSO.sub.4) due to poisoning by sulfur oxides (SOx) and that, when an air-fuel ratio of the exhaust gas is lowered after heating the nitrogen oxide absorbing material to a high temperature, the barium sulfate (BaSO.sub.4) is decomposed and desorbed in the form of a gas of sulfur oxide (SO.sub.2). Further, Japanese Unexamined Patent Publication No. 6-272538 teaches that, in order to decompose nitrogen oxides (NOx) absorbed in a nitrogen oxide absorbing material installed in an exhaust line of an engine, hydrocarbons (HC) are imperfectly burnt to produce carbon monoxide (CO) and the carbon monoxide (CO) is supplied to the nitrogen oxide absorbing material to reduce it.
However, in order to desorb a sulfur component in the form of a gas of sulfur oxide (SO.sub.2) from the nitrogen oxide absorbing material only by heating the nitrogen oxide absorbing material, it is necessary to heat the nitrogen oxide absorbing material to approximately 800 to 900.degree. C., which causes thermal deterioration of the nitrogen oxide absorbing material and is always undesirable. During heating, there occurs no decomposition of barium sulfate (BaSO.sub.4) nor desorption of sulfur oxide (SO.sub.2) until the nitrogen oxide absorbing material is heated to a high temperature even when an air-fuel ratio of exhaust gas is lowered. Further, supplying carbon monoxide (CO) to the nitrogen oxide absorbing material is made not for the sake of dissolving poisoning by sulfur oxides (SOx) but for the sake of reducing nitrogen oxides (NOx).